Since a CD (Compact Disc) is developed, capacity of an optical disc has been expanded while setting a shorter wavelength of a laser and a higher numerical aperture of an objective lens as main development targets. As a result of the emergence of a BD (Blu-ray Disc) using a blue-violet laser having a wavelength of 405 nm band and an objective lens having a numerical aperture of 0.85, the capacity of an optical disc is approaching near the limit. The reasons of the above are that, when the wavelength is 400 nm or shorter, substrate absorption becomes prominent, and that the numerical aperture of the objective lens is near 1 which is the physical limit.
In order to further increasing the capacity, as a successor of the above-described optical storage, a hologram recording/reproducing apparatus has been proposed.
A hologram recording/reproducing apparatus performs recording on the principle that signal beam and reference beam emitted from a light source are caused to interfere with each other in a recording medium, to record information three-dimensionally in the form of minute interference fringes (hologram). In the apparatus, plural sets of information can be multiply recorded in the same place of a recording medium. Therefore, the capacity can be significantly increased as compared with the two-dimensional recording of a current optical disc in which information is recorded in the form of pits or marks in a plane.
In accordance with the remarkable progress of the technical level of key components which are necessary for constructing a system of a hologram recording/reproducing apparatus, such as a spatial light modulator and an image pickup device, commercialization of a hologram recording/reproducing apparatus and widespread use subsequent thereto are becoming a real possibility.
In practical application of a hologram recording/reproducing apparatus, however, there is a difficulty of degradation of a reproduced image due to temperature difference. The difficulty is caused by a phenomenon that anisotropic thermal expansion of a hologram, and a change of the refractive index of a recording material occur with a change of the temperature, and the reference beam in reproduction does not satisfy the Bragg condition.
As a method which may solve the difficulty, there has been proposed a configuration in which the shift quantity of the reproduction wavelength is determined based on the temperature difference between recording and reproduction, and the oscillation wavelength of a variable wavelength laser is shifted. An example of such configuration is disclosed in JP-A-2006-267554 (corresponding U.S. publication is: US 2006/0232841 A1).
In the configuration disclosed in JP-A-2006-267554, although a certain improvement can be expected, however, it seems that a recording/reproducing method which is more accurate is necessary in view of configuring a stable system. Prior to compensation of a reproduced image in reproduction involving a temperature difference, therefore, a position servo control must be first accurately performed on a recorded portion. In this case, a method in which the position servo control is performed by using an external sensor may be employed. In the method, however, the stability is low in view of a temporal change of a recorded hologram, apparatus compatibility, etc. Therefore, it is preferred that a servo control is performed while a servo signal is produced by a recorded hologram itself. In the servo signal, with respect to uncertain variations such as disturbances against the temperature change and design errors, naturally, the system characteristic is requested to maintain the present status (hereinafter, this is referred to as robust). However, a simple method which can be used in a practical level has not been proposed.